The UBV photometric system, also called the Johnson system or Johnson-Morgan system, is an early well-accepted photometric system and is the system to be assumed if no other system is specified or implied.
It specifies U, B, and V bands, in turn, specifying specific B-V and U-B color indices. Magnitudes are calibrated as zero for A0 V stars unaffected by reddening.
The UBVRI photometric system, i.e., Johnson-Cousins system is an extension with red and infrared passbands, R band and I band.
The UBVI photometric system uses the B, V, R, and I from the UBVRI system, but typically with a narrower U band from a system by Thuan and Gunn.